ci: Update docker images for arm

[LucasFA]

Commit message body:

See https://github.com/cross-rs/cross/wiki/Configuration#builddockerfile
for context regarding the $CROSS_BASE_IMAGE environment variable. Here, it
resolves toghcr.io/cross-rs/armv7-unknown-linux-gnueabihf:{{cross_version}}

Relies on docker caching instead of pre-building the image.

Cross made the change to the GHCR on https://github.com/cross-rs/cross/pull/609

Add newline at EOF for some files.

It could reasonably be considered to remove the dockerfile altogether and declare the
environment variables docs and apt installation script docs, example as part of the Cross.toml file.

It's just changing a few lines more, besides deleting the dockerfile. If that's deemed a good
option, I would gladly push it to this PR branch.